🎯 Objective
To determine the iodine value of a given oil sample by Wijs method and classify the oil as drying, semi-drying, or non-drying.
📖 Principle / Theory
Iodine value (or iodine number) is defined as the number of grams of iodine absorbed by 100 g of oil. It measures the degree of unsaturation (C=C double bonds) in fatty acid chains.
In Wijs method, Wijs solution (ICl in acetic acid) reacts with double bonds: ICl + C=C → IC−CCl
Excess ICl is reacted with KI to liberate I₂, which is titrated with Na₂S₂O₃.
Iodine Value = [(V_blank − V_sample) × N_thiosulphate × 12.69] / weight of oil (g)
Classification: Drying oils (IV > 130): linseed oil | Semi-drying (100–130): cottonseed | Non-drying (< 100): coconut, castor
🧰 Apparatus Required
Iodine flask (500 mL), burette, pipette (25 mL), measuring cylinder.
🧪 Chemicals Required
Wijs solution (ICl in glacial acetic acid), KI solution (10%), N/10 Na₂S₂O₃, starch indicator, CCl₄ or chloroform.
⚗️ Procedure
- Weigh accurately ~0.2–0.3 g of oil in an iodine flask. Dissolve in 10 mL of CCl₄.
- Add 25 mL of Wijs solution using a pipette. Stopper the flask and swirl.
- Keep in dark at room temperature for 30 minutes (complete reaction).
- Add 20 mL of KI solution (10%). Add 100 mL of distilled water.
- Titrate the liberated I₂ with N/10 Na₂S₂O₃ from the burette.
- Add starch indicator when solution becomes pale yellow. Continue to colourless endpoint.
- Run a blank titration (same procedure, without oil). Record V_blank.
- Iodine Value = [(V_blank − V_sample) × 0.01 × 12.69] / weight of oil (g)
📊 Observations & Calculations
| Observation | Trial 1 | Trial 2 | Trial 3 |
|---|---|---|---|
| Initial burette reading (mL) | ______ | ______ | ______ |
| Final burette reading (mL) | ______ | ______ | ______ |
| Volume of titrant used (mL) | ______ | ______ | ______ |
| Concordant volume (mL) | ______ | ||
Volume of titrant (V) = ______ mL
Result = ______ (using appropriate formula)
✅ Result
Iodine value of the given oil = ______ g of I₂ per 100 g of oil. The oil is classified as a [drying / semi-drying / non-drying] oil.
⚠️ Precautions
- Keep flasks in the dark for exactly 30 minutes — exposure to light decomposes ICl.
- Ensure the Wijs solution is at least 2× the amount likely to be absorbed by the oil.
- Add starch indicator only near the endpoint.
- Blank and sample titrations must be done simultaneously.
❓ Viva-Voce Questions
📚 References
- IS 548 (Part 1): Methods of Sampling and Test for Oils and Fats – Iodine Value
- Vogel's Textbook of Quantitative Chemical Analysis